Hydraulic hammers of this type are previously known, for instance through GB 1478435. In this publication there is described a hammer or percussive device in which the control valve is formed as a sleeve element surrounding the hammer piston. This large diameter control valve is disadvantageous in that it is relatively heavy, which means that the valve action is relatively slow and the hammer frequency is low. The relatively large valve diameter also means long seal clearances causing a low degree of valve tightness.
In EP 335 994 there is described a hydraulic hammer having a spool type control valve. Due to a relatively small diameter this valve can be lighter and tighter. However, this valve also relies on a clearance seal to cut off the communication with one of the cylinder ports. Clearance seals are always dependent on tolerance deviations, and in order to ensure a free running valve the tolerances have to be on the large side. So, despite a relatively small diameter the clearance leakage in this type of valves is a problem and causes a limitation of the hammer efficiency.